The present invention relates to electrically conductive articles and, in particular, to articles, or bodies, having a matrix of non-conducting material, such as plastic, ceramic material, or glass, with a substantially uniform dispersion of finely divided electrically conductive material distributed therein. The electrically conductive material of the present invention is comprised of ferroalloys, silicon alloys, or mixtures thereof.
One aspect of the present invention relates to compositions of plastic or elastomeric materials, that may be shaped or molded into electrically conductive articles having improved electrical conducting and physical properties.
Another aspect of the present invention relates to the production of ceramic, cement, or glass compositions that may be shaped or molded into electrically conductive articles having improved electrical conducting and physical properties.
Most materials may be classified according to their ability to conduct or impede the flow of electrical current. Metals are generally extremely good electrical conductors, while plastic materials are generally very good insulators. Electrical conductivity may be categorized to define good conductors as having a volume resistivity of between 10.sup.-6 and 10.sup.0 ohm-centimeter; semiconductors between 10.sup.0 and 10.sup.8 ohm-centimeter; and good insulators (poor conductors) between 10.sup.8 and 10.sup.12 ohm-centimeter.
Although the present invention is directed more to electrically conductive articles which are good conductors, it will be understood that the compositions of the present invention may be easily varied, by the use of lesser amounts of conductive material, to produce articles which are semiconductors. The term "electrically conductive," as used herein, shall include both good conductors and semiconductors.
The desirability of making electrically conductive shaped articles from plastics, elastomers, ceramic and glass materials has long been recognized. Electrically conductive articles frequently include a dispersion of noble metal powders, e.g., platinum, gold, or silver. The volume resistivity of such articles is usually in the order of about 0.1 ohm-centimeter or less. Although expensive, noble metals are employed to a great extent because non-noble metal powders, such as copper, nickel, or aluminum, form high resistance surface oxides. Even with meticulous cleaning and preparation to remove the oxides from non-noble metal surfaces, the oxides reform, and, over a period of time, the conductivity of the article containing such metals decreases.
The electrical conductivity of an article containing a dispersion of electrically conductive particles depends upon the particle-to-particle contact. To be a good conductor, electric current must flow from particle to particle with the lowest amount of contact resistance possible. In the case of non-noble metal powders, the oxide coating that forms, while perhaps only a few atoms thick, has a high electrical resistivity and prevents the ready flow of current between contiguous particles.
The expense of noble metal and the disadvantages of using non-metal powders have caused a number of alternate solutions to be suggested, for example, using various mixtures of noble and non-noble metal powders; coating non-noble metal powders with a noble metal; and processes for cleaning and maintaining the powder in a free metal form even at the point of incorporation into the non-conducting matrix. However, none of these alternatives has proven satisfactory from either the initial cost, preparation expense, or performance viewpoints.
The present invention utilizes an electrical conductive material which not only has an economic advantage over noble metals, but also over non-noble metals commonly used, while providing a conductivity comparable to that of powdered copper. The present conductive materials also have brittle properties enabling them to be pulverized, or processed into powder form, easier than the metals commonly used. The electrical conductivity of the articles of the present invention are comparable to those of the non-noble metals of prior art but are much less prone to the subsequent formation of oxide coatings that increase electrical resistance.